Abrasive composition with limestone as the porosity-inducing agent



United States Patent OfiFice 3,476,537 Patented Nov. 4, 1969 US. Cl.51-296 4 Claims ABSTRACT OF THE DISCLOSURE The abrasive compositioncontains granular abrasive material, granular limestone, a granularfiller and a binder. The abrasive and limestone grains are of the sameorder in size, While the filler grains are much smaller size. The ratioby volume of abrasive and limestone in the composition approximates lessthan 50% :10%.

The present invention relates to improvements in an abrasive compositionintended for use, most commonly, in rotary grinding or finishing wheelsof a wide variety of descriptions. However, and as will be readilyappreciated by those skilled in the art, the formulation or compositionof ingredients of which such wheels are fabricated is also well adaptedfor use in abrading or finishing units of a non-rotary type, for examplein blocks, belts, sheet-s and the like.

More particularly, the invention relates to an abrasive wheelcomposition of this sort which has provision for inducing porositytherein in a novel, economical and efficient manner, with the result ofgreatly improving the performance of the wheel or other unit in aconsiderable number of respects. These may be in regard, for example, tothe length of life of the unit; to its efficiency and/r speed inremoving metal or other material from a workpiece, without burning orchipping the material; to its lowness of cost as compared withconventional, generally similar abrasive compositions; to the diminishedoperator effort involved in the use of the unit, etc. For simplicity,reference will be hereinafter made only to the use of the improvedcomposition in rotary abrasive wheels, on the understanding that it hasthe other fields of application mentioned above.

As is commonly understood, the grade or hardness of an abrasive grindingor finishing wheel is dependent upon three physical factors, allexpressed in percentage per cubic inch. These are: its porosity orpresence of voids, its total volumetric content of abrasive grains, andits total volumetric content of non-abrasive bonding or other agents.Theoretically, the most efficient grinding wheel for any given operationwould be the hardest possible wheel, as determined by the three abovefactors.

As is well known, grinding wheels remove metal or other material by aprocess of abrasive breakdown; that is, the abrasive grains mustfracture or break down, so as to present continually renewed, sharpcutting points to the material being ground. When a grain does not breakdown fast enough, it tends to become fiat-surfaced, and it generatesheat which is transferred to the material being ground, as Well as tothe wheel.

Such a wheel is then considered to be hard; and this has come about asthe result of one or both of two conhow easily a grain is crushed.

In the past, and at present, attempts have been made to alleviate thedrawback of excessive grain toughness by mechanically introducing holes,grooves or slots in the grinding surface of the wheel. This of coursecalls for specialized wheel mold parts for each wheel specification asto type and size, with resultant great increase in the cost of wheelproduction. It requires the stocking of a vast number of individuallyexpensive mold parts, a reduction of production time due to moldchangeovers, etc.

As another alternative, efforts have been made to reduce wheel toughnessby non-mechanically imparting voids or porosity in the moldingprocedure, for example, by introducing aluminum oxide bubbles in theabrasive composition. Due to the vastly different specific gravity ofthe aluminum oxide bubble, it tends to float to the top of the mix,resulting in a non-uniform product.

It is therefore a general object of the present invention to provide anabrasive block composition for use in grinding and finishing wheels orother abrasive-type material removing devices, which composition hasporosity intentionally introduced therein in a novel and improvedmanner, thus vastly to improve the wheels performance in respect to thevarious considerations of length of life, grinding capacity andefficiency, and the like, mentioned above.

More specifically, porosity is induced by the addition to the abrasivecomposition or formulation of a random distribution of a granular agentapproximating the abrasive grains in grit size, and softer than theabrasive grains. In this connection it is to be understood that the addition in question is by no means a filler, or equivalent of a filler, onany basis, i.e., its intended function, its particle size or itsidentity. It is to be emphasized that the purpose and action of theporosity inducing agent is to replace an approximately equal number ofthe abrasive grains of a harder nature. Thus, the agent stabilizes theabrasive mix in curing, then wears, decrepitates, decomposes ordissolves during the grinding process in a way to enable the abrasivegrains to serve more efficiently individually as cutters.

In further accordance with the invention, the porosity inducing agentreferred to above is preferably one which is widely available at verylow cost, as compared with that of the abrasive grain material. I findthat a limestone of onetype or another, for example, calcspar (calciumcarbonate) or dolomite (calcium-magnesium carbonate), is ideallysuitable from the standpoints of commercial availability and low cost,wide selection of grain size, etc. It is to be clearly understood thatthe limestone (CaCO intended for use in the improved composition is tobe sharply distinguished from the lime (CaO) conventionally used ingrinding Wheels and the like. Other possible selections for the porosityinducing agent may be made from natural or activated bauxite, andminerals such as olivine, gypsum, chromite, coquimbite, pyrolusite,molybdenite, galena, halite, and the like, as well as a variety ofmanufactured products for a similar purpose.

It will be noted that the materials referred to above vary quite widelyin hardness on the Mob and Knopp scales, i.e., from Moh Nos. 1-3 to ashigh as 6 or 7. Accordingly, the selection will be governed to aconsiderable extent by the selection of the particular granular abrasivewhich is used, as well as on the basis of the grinding operationinvolved, in point of kind and amount of material to be removed, desiredspeed, finish, life, etc.

The effect of the porosity introducing agents referred to above, in arandom distribution in the abrasive mass of cutting grains, binder,filler, etc., is to offer continuing relief for a considerable quantityof the abrasive grains, as the porosity inducing agent mechanicallywears away, decrepitates, decomposes or dissolves under the effectsinherent in the grinding operation.

As for the granular abrasive component of the wheel, it may be in one ofvarious types commonly employed, typical of which are hereinaftermentioned in tabulations of volume structure data taken from certaincomparative tests.

Another object of the invention is to provide abrasive materialformulations incorporating one or another type of porostiy inducingagent, in which the performance qualities, capacity and life of a wheelfabricated therefrom may be readily and extensively controlled by simplychanging from one porosity agent to another, by mixing different agents,by altering the total percent content of the agent in the mixture, byvarying its granular size, etc.

A further object of the invention is to provide a porosity-increasedcomposition which is capable of being employed in grinding wheels or thelike for a large number of different types of operation. Examples areface grinding, foundry snagging, roll, centerless or off-hand grinding,segmental disc wheel grinding, cylindrical grinding, internal grinding,ceramic tile grinding and the like; and in which many gradations ofrough or finished surface are required.

For example, the improved wheel of the invention has been found veryeffective in the end grinding of coiled torsion springs ranging widelyin diameter; and this type of operation will prove useful in explainingmy theory in regard to the improved performance of the wheel.

Thus, a conventional grinding wheel will display, in microscopic crosssection, a more or less uniform dis tribution of abrasive grains,resinous binder and natural voids, in what may roughly be considered acheckerboard distribution, for the purpose of discussion. Assume that inthe improved wheel of the invention, a corresponding cross section willexhibit a generally, but not exactly, similar distribution of abrasivegrains, binder and natural voids or porosity plus, in addition, a randomplacement of porosity inducing granules.

It will therefore be seen that in at least some partial cross sectionalarea, smaller or larger, of the improved wheel composition there will bea distribution of abrasive grains, binder, agent and natural voids whichis the same as that of the conventional wheel section, lacking entirelythe porosity inducing grains contemplated by the invention. In anotherportion, or the remainder, of the cross sectional area, the porosityinducing granules will be found in their random distribution.

It follows, naturally, that the two wheels under consideration will havesubstantially identical cutting characteristics, being of equalhardness, at their areas of corresponding distribution of abrasivegrains, binder and natural voids. By the same token, the wheel of theinvention is softer in the area of non-corresponding distribution. Theadvantage is that the porosity imparting granules, having firstmechanically stabilized the wheel composition in the molding and curingthereof, will in turn gradually wear out, decrepitate, decompose ordissolve in the cutting operation. Accordingly, a certain percentage ofthe abrasive grains, deprived of lateral support, will more readily chipor break down progressively in a desirable manner. The abrasive grainsare made more eflicient as cutting tools, in the accepted sense referredto above, and last longer. The improved wheel will remove more than, orat least as much metal as, the conventional wheel, and usually withoutburning.

Now, in reference to the instanced grinding of coiled torsion springs,it is obvious that a coil with a small wire diameter will be subject toa relatively high unit pressure at its relatively low total area ofengagement or contact sweep by the wheel. On the other hand, a spring ofconsiderably larger wire diameter will be subjected to a lower unitpressure against the wheel, but by way of compensation is operated onwith greater cutting efiiciency due to the increased presence of theporosity inducingagent in a larger area of sweep by the wheel, i.e., anarea in which more of the porosity inducing granules or particles arepresent. The result is a uniformly good and efficient grinding of thesmaller and larger size springs. Essentially the effect is the same asin the case in which porosity is incorporated in the wheel by theproduction molding of slots, recesses, etc. therein a mechanical way,but in accordance with the invention, the full and much less costlyequivalent of such holes or recesses is bad in a vastly different way.

The tabulations in terms of volume structure which follow comparevarious formulations of the improved wheel of the invention withconventional grinding wheels, and are based on data as to comparativeperformance on several different types of grinding operation, also ondifferent types of work. The wheels of the respective sets undercomparison were intentionally and carefully fabricated to have as muchin common as was practically possible, for example, in regard topercentage by volume formulation.

The powdered and liquid binder and filler components were of well knownand widely available types. Actually, a powdered two-stage thermosettingphenolic resin of the phenol formaldehyde type, known as Varcum 3030 andcommonly used to bond grinding wheel abrasive grains, was the dry binderin each instance. Other thermosetting resins or the like would beequivalents, as well as vitreous-type binders of one sort or anothersuch as are commonly used in the industry. A one-step water solublephenolic resin, known as Varcum 8121, was used as a wetting agent orliquid resin, and would have its equivalent in furfural, furfuralalcohol, etc. Cryolite, a double fluoride of sodium and aluminum, alsoreferred to as ice stone was the filler, one commonly employed in thegrinding wheel industry. Equivalents might be lime, calcium oxide,potassium sulfate, iron sulfide, wollastonite, fine emery, etc.

It is here well to emphasize that such fillers are commonly used in theindustry in #200 and finer mesh sizes, (although they have been used insizes as great as #46 and #60 mesh). The fillers contemplated by me areof much finer nature than the grit sizes contemplated for the importantporosity inducing agent of the improved composition, which range in theneighborhood of #10 through #24. The function of that agent is in nosense merely the function of a filler.

Limestone, as used for the porosity inducing agent of the improvedcomposition is equally sharply distinguishable from lime, in thecontemplation of the invention.

As for the abrasive grain composition of the wheels under comparison,the aluminum oxides were, generally speaking, fused bauxite, withadditions of zirconia and/or titania in some instances for additionaltoughness. Black silicon car-bide was employed in some instances, alsofor a desired degree of toughness. Finally all of the wheels inquestion, improved and conventional, had their ingredients mixed andbaked in an industrially well-known procedure, identical in the case ofeach of the respective comparative formulations.

CONVENTIONAL WHEEL A IMPROVED WHEEL B These two wheels, both in a 30"outer diameter, a 12 inch inner or hole diameter, and a ,4 inch axiallength, were employed in a horizontal spindle disc operation, usinggrinding fluid, on coiled springs. The composition of the respectivewheels was as follows:

Conventional wheel A Volume structure: Percent Aluminum oxide#20 grit S2Limestone None Powdered resin 12 Liquid resin 4 Porosity 32 Density,.079549 #/in.

Improved wheel B Volume structure: Percent Aluminum oxide-#20 grit (23%decrease) 40 Limestone-# 14 and #16 grit Powdered resin 18 Liquid resin6 Porosity 26 Density, .077547 #/in.

Conventional wheel A removed 530 pounds of metal per set of two wheels,with frequent bad burning of the workpieces. Improved wheel B, on theother hand, removed 655 pounds per two wheel set with no burning of thematerial. This is a 22.6% improvement in output capacity, without needfor regrinding of burned pieces and possible scrap loss.

CONVENTIONAL WHEEL C- IMPROVED WHEEL D These wheels wereperformance-compared at 9500 s.f.p.m. in a swing frame foundry grindingoperatlon on annealed malleable iron. The composition of these twowheels was as follows:

Conventional wheel C Volume structure: Percent Aluminum oxide (40%zirconia)--#14, #16

Density, .099882 #/in.

Wheel C ground 19,800 pounds of malleable castings, ranging in weightfrom 60 to 200 pounds, with riser or gate dimensions to be removed whichranged from one cubic inch to five cubic inches. As operated on the samemachines by the same operators, improved wheel D ground 54,000 pounds ofcastings, representing a produc tion improvement factor of about 172%.

CONVENTIONAL WHEEL EIMPROVED WHEEL F These wheels, in dimensions of sixinch CD. by five/ eighth inch ID. by one inch thickness, were used at9500 s.f.p.m. in the offhand portable grinding of stainless steel. Therespective compositions of the wheels in question was as follows:

Conventional wheel E Density, .084831 #/in.

Conclusions in a comparison operation of this type are based primarilyon the opinion of the operator or operators,'rather' than uponproduction records kept at the plant, as in the case of the first twocomparative tabulations discused above, which records determine thepolicy of the plant in purchasing grinding wheels.

On the basis of operator opinion (also referred to commonly in decisionsregarding reordering) conventional wheel E was rejected for the reasonthat it required more operator effort and occasionally burnedworkpieces. On the other hand, the improved wheel F cut at a much morerapid rate than wheel E, requiring less operator effort to perform theoperation. Moreover, wheel F was a very free cutting one, while having alonger life of operation than the conventional wheel in the sameoperation.

CONVENTIONAL WHEELS G AND H-- IMPROVED WHEEL I These three wheels, alldimensioned. 30" CD. by 6'' ID. by 3" thickness, were disc wheels run at7000 s.f.p.m. in the grinding of vitrified and glazed tile products fordecorative or conventional purposes. Conventional wheel G was anentirely standard one, which had in the past done an apparently verysatisfactory job. Wheel H was also of generally conventionalcomposition, differing slightly from wheel G in regard to percentageporosity and resin content. However, in these porosity and resinfactors, wheel H was identical to improved wheel I. It is to be notedthat the abrasive material employed in wheel I was of somewhat finermesh or grit size than in the case of wheels G and H, with what might beexpected to result in a lessened material removing capacity. Therespective compositions were as follows:

Conventional wheel G Density, .067462 #/in.

Improved wheel I Volume structure: Percent Black silicon carbide#30 grit(19.3% decrease) 42 Limestone-#20 & #24 grit 10 Powdered resin 12 Liquidresin 4 Porosity 32 Density, .065612 #/in.

CONVENTIONAL WHEEL J--IMPROVED WHEEL K The wheels compared were Type 11cup wheels (6/4% by 2 /2" by /6"11). The operation was portable oif-handgrinding at 9500 s.f.p.m. of Super Stone Metal, a combination ofmanganese, aluminum, nickel and bronze. The compositions of wheels I andK are as follows:

Conventional wheel J Volume structure: Percent Black silicon carbide#12, #14 and #16 grit--- 52 Limestone Powdered resin 15 Liquid resinPorosity 28 Density, .069252 #/in.

Improved wheel K Volume structure: Percent Black silicon carbide#12, #14and #16 grit (14.5% decrease) 46 Limestone# grit 10 Powdered resin 14Liquid resin 4 Porosity 26 Density, .071145 #/in.

The operators opinion as to the cutting etliciency and overall life ofthe wheel, i.e., the time required to wear a wheel down to a no longeruseful size, is the performance standard on this type of grinding job.

Conventional wheel I was satisfactory (performancewise) from theoperators standpoint. However, it had a useful life of approximately 1hour and 45 minutes. Improved Wheel K was equally satisfactory inperformance to operator, but it lasted with etficiency for approximately2 hours and 30 minutes, a life increase of 45 minutes. Thus, by thereplacement of six parts of abrasive material by limestone, efiiciencywas increased by approximately 43% in improved wheel K.

One of the important conclusions to be drawn from the foregoingtabulations is that in all of the five instances, the improved wheelenables a very significant diminution in the amount of expensiveabrasive or cutting agent which was employed, in the neighborhood ofabout 14%24%.

Equally significant are the improvements in output capacity; thediminution of operator effort; the minimizing of burned workpieces,requiring reworking with the possibility of not meeting tolerance; thelonger effective life, etc.

It is to be noted that the granular size of the limestone porosityinducing material is approximately of the same order as that of thegranular abrasive material,

i.e., in a range of #10 mesh-#30 mesh, actually #10 mesh-#24 mesh forthe limetsone. 'In other words, the porosity inducing granules, asrandom distributed in the abrasive composition as a whole in anapproximate ratio by volume of less than 50% abrasive grains to 10%limestone grains, are of suflicient bulk to sustain the granularabrasive during mold-ing and baking or curing, yet will readily andgradually disappear as wheel wear proceeds, enabling the abrasive grainsto present newly restored and sharp cutting points. The sizedifferential relationship is of an entirely different order than thatexisting in regard to the abrasive grains and filter material, thelatter of which is often of flour-like proportion, in a size of aslittle as #400 mesh.

Such fillers are commonly selected and used on the basis of one chemicalcharacteristic or another, whereas this is not a factor in the selectionof the porosity affording agent of the improved composition. In fact, inaddition to other attractive features of granular limestone, asdiscussed above, one of its desirable aspects in the light of thepresent invention is that it is substantially free of chemicalimpurities, so that its performance of its intended function is entirelypredictable.

Actually, the porosity inducing agents contemplated by the inventionlose their porosity inducing characteristic and capability in the finergranular sizes.

As indicated above, the hardness of the granular limestone and otheroptional porosity imparting agents may range considerably, i.e., from 1to 6 or 7 on the Mob scale. The abrasive material will normally have aMob number of 9 or 9.5, although hardnesses of as little as 6 or 7 havebeen employed. Accordingly, it is not possible to express any particularrange of hardness numbers for the porosity agents, its selectiondepending, as it does, on the selection of the abrasive agent, and thisin turn depending upon a large number of considerations, such as thematerial of the workpiece, the fineness of finish desired, etc.

What I claim as my invention is:

1. A grinding member which, in final set condition ready for use,includes an abrasive composition comprising solid granular abrasivematerial bonded in combination with solid granular porosity inducingmaterial, the porosity inducing material being essentially limestone,being of a hardness substantially less than that of the abrasivematerial, a granular filler material, and a thermosetting resin bindermaterial, the grains or granules of the respective abrasive and porosityinducing materials being of approximately the same order in size, andmuch greater in size than those of the granular filler material.

2. A grinding member which, in final set condition ready for use,includes an abrasive composition comprisin-g solid granular abrasivematerial bonded in combination with solid granular porosity inducingmaterial, the porosity inducing material being essentially limestone,being of a hardness substantially less than that of the abrasivematerial, a granular filler material, and a thermosetting resin bindermaterial, the grains or granules of the respective abrasive and porosityinducing materials being of approximately the same order in size, andmuch greater in size than those of the granular filler material, thegrains or granules of the abrasive material and porosity inducingmaterial being in the approximate range of #10 mesh-#30 mesh in size.

3. A grinding member which, in final set condition ready for use,includes an abrasive composition comprising solid granular abrasivematerial bonded in combination with solid granular porosity inducingmaterial, the porosity inducing material being essentially limest ne,being of a hardness substantially less than that of the abrasivematerial, a granular filler material, and a thermosetting resin bindermaterial, the grains or granules of the respective abrasive and porosityinducing materials being of approximately the same order in size, andmuch greater in size than those of the granular filler material, thelimestone being of a granule size in the range of approximately #10mesh#24 mesh.

4. A grinding member which, in final set condition ready for use,includes an abrasive composition comprising solid granular abrasivematerial bonded in combination with solid granular porosity inducingmaterial, the porosity inducing material being essentially limestone,being of a hardness substantially less than that of the abrasivematerial, a granular filler material, and a thermosetting resin bindermaterial, the grains or granules of the respective abrasive and porosityinducing materials being of approximately the same order in size, andmuch greater in size than those of the granular filler material, theabrasive and porosity inducing materials being in an References CitedUNITED STATES PATENTS Halstead 264'12 Taylor 106-422 Kistler 51298Kistler 51-298 Wooddell et al 51298 Robie 51-298 Cofran 51296 DONALD J.ARNOLD, Primary Examiner approximate ratio of less than 50%:10% byvolume in 15 51 298, 308

the composition as a Whole.

US. Cl. X.R.

